The present invention relates to a process for the polymerization of acrylic monomers and optionally of vinyl comonomers using an initiator of formula (I): EQU R--M (I)
in which M denotes a metal chosen from the group selected from alkali metals and alkaline earth metals and R denotes a straight-chain or branched alkyl radical containing 2 to 6 carbon atoms or an aryl radical.
Processes for the polymerization of acrylic or methacrylic monomers, such as alkyl acrylates and methacrylates, as well as processes for the copolymerization of acrylic or methacrylic monomers with vinyl comonomers, in the presence of various initiators, especially those of formula (I) given above are known. Examples of such initiators are sec-buthyllithium, napthalenesodium, 1,4-disodio-1,1,4,4-tetraphenylbutane, diphenylmethylpotassium, diphenylmethylsodium, 1'-methylstyryllithium, 1,1-diphenyl-3-methylpentyllithium and others, such as tertiary alcoholates of lithium and compounds containing trimethylsilyl groups.
In polymerizing acrylic monomers and optionally non-acrylic comonomers, it is known to use additives or various catalysts, especially cocatalysts or additives containing, for example, sources of fluoride, cyanide or acid ions or consisting of a Lewis acid. Finally, the use of an alkali metal or alkaline earth metal salt of an inorganic acid is known from European Patent Application No. 0,185,641.
On the other hand, French Patent No. 2,201,304 describes the polymerization of vinyl monomers, especially of alkyl or cycloalkyl acrylates and methacrylates, with anionic initiators in an organic solvent medium and in the presence of a nitrogenous macroheterocyclic complexing agent in a quantity that is at least equimolar relative to the anionic initiator. This patent further describes the polymerization of methyl methacrylate at -78.degree. C. and +25.degree. C. in benzene or tetrahydrofuran. The French certificate of addition No. 2,398,079, attached to this patent, describes a variation in which the polymerization occurs in the absence of a solvent. In the case of methyl methacrylate, the polymerization is carried out at -40.degree. C. and -80.degree. C. respectively and leads to polymers having number average molecular masses of 150,000 and 400,000, respectively.